1. Nature of The Invention
This invention is concerned with reducing the wax content of distillate hydrocarbon fractions by conversion of straight or slightly branched paraffin hydrocarbons contained therein.
2. Prior Art
Distillates obtained from crude oil, such as gas oils have been processed heretofor to produce fuel oil products, including home heating oil, diesel fuel, furnace oil and the like. Specifications for these products normally include a requirement that the pour point may not exceed a certain maximum value. In some instances it is necessary to subject these distillate fuels to additional processing whose principle purpose is to reduce the pour point of the feed stream.
One such process is catalytic hydrodewaxing in which gas oil is contacted with hydrogen and a shape selective catalyst adapted to selectively crack or hydrocrack the paraffinic molecules in the gas oil. Initially the catalysts used were those zeolite cracking catalysts which had pore openings sized so that they would admit and crack only normal paraffins and exclude all the other gas oil components. An example is an erionite-type zeolite. More recently U.S. Pat. No. RE 28,398 has disclosed an improvement in this process where ZSM-5 type zeolites are used in place of the previously used erionite-type cracking catalyst. U.S. Pat. No. RE 28,398 is incorporated herein by reference. This process permits lowering of the gas oil pour point in a very efficient manner. The product of this hydrodewaxing process may be suitably fractionated to produce high yields of dewaxed gas oil boiling in the same range as the feed.
In a hydrodewaxing operation as presently practiced, the vaporized distillate is introduced into a bed of ZSM-5 type crystalline zeolite catalyst at a temperature maintained within the range of 650.degree. F. to 1000.degree. F., a pressure of 100 to 3000 psig and a liquid hourly space velocity of 0.1 to 10 and a hydrogen/hydrocarbon mole ratio between 1 and 20. The hydrodewaxing reaction within the catalytic bed is an endothermic reaction. It is thus necessary ordinarily to superheat the change before admitting it to the catalyst bed. The amount of heat available is controlled by the heat capacity of the super heated reactants and/or inerts used. In an adiabatic commercial unit the temperature drop throughout the reaction bed may be as high as 20.degree. to 50.degree. F. As feeds of an increasing wax content are hydrodewaxed in this process, endotherms or temperature drops as high as 130.degree. F. may be anticipated. As the temperature declines in the reaction bed, the rate of reaction and consequently the rate of dewaxing of the oil diminishes
One object of this invention is to provide an improved catalytic dewaxing process wherein the temperature decline in the catalyst bed is minimized. Still another object of this invention is to provide an improved catalytic hydrodewaxing process.